1. Field of the Invention
This invention relates to conductive polyolefin composites. More particularly, it relates to composites containing polyolefin, conductive carbon and, optionally and preferably, an inorganic filler, and to a process for electroplating molded objects of these composites.
2. Description of the Prior Art
The utility of organic polymers has been broadened in recent years to the degree that rigid polymers such as the nylons, ABS (acrylonitrile/butadiene/styrene) and polyacetal resins have begun to replace the more conventional metal, wood and ceramic materials. Methods have been developed for increasing the rigidity of lower cost polymers such as polyolefins, mostly by filling them with finely divided solids or fibrous fillers, thereby making them candidates for many of these applications.
One such method that is particularly advantageous is described in German Patent Publication 2,459,118. Briefly, in this method a solid, homogeneous, highly filled composite of polyolefin, particularly polyethylene, with an inorganic filler is made by polymerizing the olefin onto the surface of the filler, the filler having had interacted at its surface a catalytically active transition metal species. The resulting composites can be molded to articles of superior physical properties.
A particular field in which organic polymers, particularly ABS, have partly replaced metals is that of shaped articles coated with decorative metal surfaces, such as automobile grilles, wheel covers, appliance bases, and knobs. ABS, however, has at least two disadvantages in this use. It is relatively expensive in comparison with polymers such as lower polyolefins. In addition, in common with other organic polymers, it requires a number of laborious and expensive treatment steps to make the surfaces of its molded objects receptive to electroplating, the common method of applying decorative metal coatings.
Polyolefins would be excellent substrates for electroplating because of their high toughness, low price, and ready availability. In general, however, a number of both mechanical and chemical treatments are required to satisfactorily electroplate polyolefins; see W. Goldie, "Metallic Coating of Plastics", Vol. II, pages B 367-372, Electrochemical Publications Ltd., 1969.
One process of electroplating certain polypropylene compositions without the use of a prior electroless plating sequence is described by Adelman in U.S. Pat. No. 4,002,595. In this process, a polypropylene molded object containing polypropylene, polyethylene, a rubber, carbon black and optionally silicate mineral is preconditioned in a five to eight step etching and rinsing sequence involving chromic acid followed by electroplating. It would be desirable to provide a process which is applicable to polyethylene and polypropylene molded objects which does not involve the pollution problems and expense associated with the use of chromic acid etchants.
Another way of rendering polymers such as polyethylene electroplatable is taught by Amos in U.S. Pat. No. 3,416,992. This technique involves, for example, the mixing and homogeneous blending of microfine polyethylene powder with expanded graphite, compressing and baking the mixture and plating the resulting slugs. The expanded graphite is prepared by treating natural graphite having a particle size of 10 to 325 mesh U.S. Standard Sieve (2000 to 44 microns) with concentrated acid, washing and rapidly heating in a hydrocarbon flame. It is stated that these resin/expanded graphite slugs are plated much more effectively than slugs prepared with powdered carbon. Since powdered carbon is less expensive than expanded graphite, it would be desirable to provide a way to effectively electroplate composites containing powdered carbon.